I already have a lot of different VCOs, from the ultra precise MOTM-300
to the lush, but not so well-tracking VCOs of my EMS-Synthi clone. Each
VCO has it's benefits, the CEM3340's (partly under autotune control) are
the "workhorses" in the OB-8, the Prophet 5 and my own JH-3, the Yamaha
CS-oscillators are more on the "temperamental" side, but still playable
without autotune (the 4 ones in my CS-50 stay better in tune than the 8
ones in my CS-60 ...). The precision of the MOTM VCO is invaluable for
complex audio range modulation patches. I've never heard better drones
than from three EMS VCOs running at almost the same frequency, and beating
against each other in an ever changing pattern.
My goal was to build a set of VCOs that have the untamed bass range
power of early EMS and Moog VCOs, but which are tracking a keyboard voltage
over 5 or more octaves nevertheless. I found that "untamed" Beating in
the bass range and controlled beating in higher octaves is not possible
with standard exponential 1V/Oct oscillators. A good part of that special
sound of early Moog and EMS oscillators is not because of any "randomness",
"unstability", "instability" or "noisyness", as so often is said. A good
deal of their behavior is because of that, but it is not the whole
story. There are also some very deterministic factors in these old circuits
which have been unpleasant side effects for the designers back then, but
which are worth a closer analysis when we're designing a musical VCO today.
This is implemented in form of three "linear detune" potentiometers on
the JH-5A VCOs.

Either one shared, or 3 individual potentiometers for Initial Pulse Width

Analysis: What certain VCOs have in common

Early Moog Modular VCOs: The have a linear (Hz/V) oscillator
core, driven by an exponential converter. An offset voltage at the
point where the two parts interface will cause what I've called "linear
detunig" - the possibility of being slightly out of tune that increases
towards the bass range, where offset voltages have more influence than
on the upper range. (Later Moog Modulars don't have this: They have an
exponential VCO core, i.e. the linear ramping of the integration
capacitor is controlled by an exponential current source. Offset
voltages don't matter in that context.)

Moog Taurus Pedals also have a linear (Hz/V) oscillator core,
driven by an exponentially scalled keyboard divider. Offset voltages
are a source of possible "linear detuning", just like in the early Moog
Modular VCOs. (Taurus II doesn't have this - it has plain exponential
V/Oct VCOs.)

The Yamaha CS-80 has linear (Hz/V) oscillators. Offset voltages
are a big issue along the CV path in that synth, and must be carefully
trimmed. What portion of it is left untrimmed, will result in "linear
detuning". It makes a Cs-80 with its 2 VCOs per voice sound huge - the smaller ones, less so.

The
Korg Trident also has linear VCOs, with trimmers for offset / low range
tuning. It has very animated bass notes (but the power of its bass
sounds is spoilt a little by its noisy VCAs, IMO.)

The EMS VCS3 makes increadibly good drones (listen to Pink
Floyd's "Obscured by Clouds" album!); it shows a very similar behaviour
in the low range as the synths mentioned before - but it has
exponential, V/Oct VCOs! But on closer inspection of the linear part
(after the exponential current source), you find an offset like on Hz/V
VCOs, but not in form of an offset voltage, but an offset current
instead! The reason is the input bias current of the integrator, built
from discrete bipolar transitors. (No FET input opamp here, and no
current compensation technique as in low bias current bipolar opamps,
either.)

Now as my goal was to implement that feature in a standard,
exponential V/Oct VCO, I could model that bias current effect of the
VCS3, using a FET-input opamp (almost zero bias current), plus a
variable bias current source. So with the turn of a potentiometer, I
can apply as much "linear detuning" effect as I want. (I have published
a linear detuning circuit many years ago, whcih works on Oberheim
SEM-style VCOs that use passive integration and a voltage follower;
buit it's much easier to implement on an inverting opamp integrator -
it just takes 3 resistors.)

Drift and Noise and all that stuff

Every few years there seems to be a heated debate how "stable" a good-sounding VCO should be, or shouldn't be.
I never quite understood how one can make an almost religious question
out of this. IMO, the situation is quite easy: Find which factors cause
these random changes in a VCO (there aren't many possibilities!), and
then decide if you either

want to get rid of them, or

add them externally, or

deliberately keep them in your VCO.

VCOs with design philosophy 1 are more expensive, can be used in
applications where stability is important, and you can always do Number
2.
I could have built a VCO like this, and added the linear detune feature
there. But I decided to go for Number 3 in the "Living VCOs" project.
Asuming those who buy them want to play animated 3-VCO-sounds in the
first place, I kept everything as simple and unexpensive as possible,
and I've chosen the same noisy 4-transitor exponential converter that
EMS used in the VCS3. I'm not overly scientific about it - I just like
the sound of it.
To demonstrate the wandering frequency, I've made a short video, taken from the oscilloscope (external link).
The setup was like this:
Using only two VCOs, nearly in unison at a rather high pitch, one going
to channel 1 of the scope, the other to channel 2. The scope is
triggered from channel 1, so one of the VCOs seems to stand still. (Of
course in reality it doesn't but as others have shown, it's quite
difficult and possibly very misleading to try and show the wandering
pitch of a single VCO with a
scope.) So when you see the wave of VCO 2 wandering, you don't
see the momentary pitch of VCO, but you see it relative to VCO 1. Which
is all that really matters for the sound, IMO.)
I should also mention that this effect is more important for the
animation of 2-VCO-Sounds. When using 3 VCOs, there are very complex
beating patterns even when the individual VCOs are perfectly staying at
one frequency. So I think in my 3 VCOs + Oscillator Driver Setup, that
subtle built-in random modulation is a nice add-on, while the real
interesting feature is the low range animation - the "linear detuning"
stuff.

Like the Yamaha CS-80, the Living VCOs have a limited pulse width. Even
with strong pulse width modulation, you never "loose" the VCO signal,
because you never get down to 0% or up to 100% pulse width. What may
sound like a limitation at first, actually opens the possiblitity of
"overmodulation": You can modulate the pulse width with a triangle LFO
of rather slow rate, and with a depth that would normally be bigger
that 100% modulation index. As a result, the modulation is clipped,
becomes trapeuoid-shaped. Musically, this is like a periodic "push" of
the modulated oscillator's pitch - something that sounds less detuned
than ordinary PWM and allows a sonically very rich modulation.Single-VCO-PWM-demo (One VCO thru wide open filter + VCA + reverb)

What I provide is just the PCB and some ideas how to hook it up.
Whether you want modulation input attenuators or not, individual pulse
width setting or one setting shared among 3 VCOs, is up to you. Same
for output attenuators. The waveform from the 3 VCOs are roughly
+/-1.2V to be compatible with Moog systems. The board also contains 3
Amplifiers to boost thsi up to the MOTM (and others) standard level of
+/-5V. The rightmost pin of the SAW_n connectors carries a 0 ... 10V
saw from the VCO core; there you can connect waveshapers for triangle
and sine and so on, if you like. Because of the many possible choices
the following options are just given as examples. Of course you can
combine ideas from different options, and make your own personalized
version.

Calibration is quite similar to what you have on other VCOs.S - Scale. Here you
adjust the 1V/Oct tracking. Apply a CV from your Keyboard or
Midi->CV Converter. Connect a guitar tuner or frequency counter to
the VCO output and adjust "S" until 1 octave on the keyboard is exactly
1 octave of VCO frequency. F - Frequency. Here
you can adjust the absolute frequency (not the scale). You can set it
to something like 16Hz (or whatever else you prefer) for the ccw end
position of the 10-Turn front panel "Frequency" control.H - High frequency tracking.
Honestly: i didn't even adjust this in my prototype - I just left it in
mid position. If you need to, you can fine tune the octave tracking for
higher frequencies here.PW1(2)(3)Adj - Pulse width adjust. With the front panel Pulse Width knob in 12 o'clock position, adjust the trimmer for 50% pulse width.